If the circuit transmits successfully when you short the non-ground side of C9 to the non-ground side of D2, then I suggest you check/try the following:

1. Check that R5, R8 and R9 have the correct value.

2. Check transistor T2.

3. Check that D3 is the right way round and the correct type (i.e. a signal diode not a power diode).

4. Temporarily replace your 2-capacitor C9 arrangement with a single capacitor, positive side to ground (no need to get a 10uF cap if you don't have one, 22uF should do). Then make sure you power up the PC and connect the RS232 cable BEFORE connecting power to the Arduino, and likewise remove power from the Arduino before disconnecting the RS232 cable. This should avoid a reverse voltage on C9.

If (4) fixes it and you still can't get a non-polar electrolytic, you could try a 1uF or greater metal film capacitor, if you can find one - that should work at high baud rates.

PS - what country are you in?

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

1. R5,R8 are correct. R9 is 4k instead of 4.7k, when I was adding components I had the mounting diagram with me. And it is not very clearly mentioned on it. I thought it was 4k. Now when I see the schematic carefully, I noticed it. I will replace it today.

2. I checked the transistor T2 and it is working fine. I checked the connectivity of EB and CB terminals with a multimeter bothways.

3.D3 is correctly connected and it is a 4148 switching diode.

4. I did check that in the morning, and it still didn't work. But I didn't do it in that sequence, I will try again.

4k is an odd value - it's hard to find resistors with that value, so I suggest you check it with a multimeter. If it really is 4K instead of 4k7, I wouldn't expect it to make a significant difference to the operation of the circuit.

Here is a test you could try:

- lift the end of R5 that is connected to the TxD pin of the mcu

- with the RS232 port connected to the PC and the Severino powered up, check the voltage on the non-ground side of C9. It should be around -10v. Then check the voltage on pin 2 of the RS232 port (= collector of T2). It should be between -10v and -6v.

- connect the end of R5 that you lifted to ground. Check the voltages again. The non-ground side of C9 should still be around -10v or perhaps nearer -8v this time, but pin 2 of the RS232 port should be nearly +5v.

Let me know the 4 voltages you read.

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

4k is an odd value - it's hard to find resistors with that value, so I suggest you check it with a multimeter. If it really is 4K instead of 4k7, I wouldn't expect it to make a significant difference to the operation of the circuit.

It measures 3.86 K ohms.

Quote

with the RS232 port connected to the PC and the Severino powered up, check the voltage on the non-ground side of C9. It should be around -10v. Then check the voltage on pin 2 of the RS232 port (= collector of T2). It should be between -10v and -6v.

C9== -9.06VPin 2 == -5.02 V

Quote

connect the end of R5 that you lifted to ground. Check the voltages again. The non-ground side of C9 should still be around -10v or perhaps nearer -8v this time, but pin 2 of the RS232 port should be nearly +5v.

That 0.19v is the problem. Either you have emitter and collector of the bc557 transposed, or the resistor you lifted has the wrong value, or the transistor is faulty. Regarding pinout, I can't read one of those datasheets on the mobile I am sending this from, however some transistors come in different pinouts with different suffixes to the main part number.

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

I lifted the end that was NOT connected to the base of T2, I replaced the capacitors because they seemed like shorting with another similar pair, changed the transistors, changed the 3k9 to 4k7, changed R5, but everything is exactly the same.

And exams were fine. Thanks for your concern

PS. I think I should try reversing the pnp before giving it up. Maybe it is the odd type transistor with opposite pinout.

This was it. The pinout of the transistor was opposite, and it did not mention the company or anything to distinguish it with other bc557s. I think companies that don't stick to standard pinout and don't write anything on the IC to distinguish it should be banned!!! :0 :0 :0

And I cannot thank you enough for it. I really appreciate the help you and other members of this forum offer to everyone.

At the moment serial communication is going great. I am just having some troubles with the bootloader, I have burnt it, but right now its not working. But I think it is just a software problem, which I will sort out hopefully.